Mechanical circulatory support has become widely applied in recent years. The number of patients treated with rotary blood pumps now exceeds the number of patients treated with heart transplant. Hydrodynamic rotary blood pumps include axial flow, mixed flow, centrifugal flow, and hybrid configurations. The mechanical principles of operation of these pumps have been developed, and their design and structure has largely been determined, by the need to meet the hemodynamic requirements of heart failure patients while avoiding excessive blood damage, and avoiding thrombus formation or bleeding resulting in the need for anticoagulation.
The Jarvik 2000 is an axial flow ventricular assist device that has sustained patients for more than seven years. We have experience with more than 300 patient years of support. We have learned that patient safety is enhanced if the patient has enough residual pumping function of their natural heart to maintain life long enough to switch to backup external equipment if the pump stops due to accidental damage to the external control system, batteries, or cables. When patients are supported for many years their external heart assist system components are exposed to all sorts of accidents that should be viewed as inevitable. For example, a Jarvik 2000 patient was carrying his control system and batteries in a camera bag worn on a shoulder strap. His mechanical heart within his chest was powered by a cable that had a connector attached where the wires exit the skin. When the patient was out shopping, a purse snatcher grabbed the camera bag off his shoulder and ran away with it. The cable pulled free of the connector and the pump then stopped because it lost power. Fortunately, the control system has a loud alarm that sounds if the pump stops. This frightened the purse snatcher, who threw the camera bag on the ground and ran away. Because the Jarvik 2000 has low regurgitant flow when turned off, the patient retained consciousness and was able to walk about a block to the place where the bag was left, and reconnect his artificial heart. If any rotary blood pump has high regurgitant flow, this will overwhelm the ability of the patient's natural heart to maintain sufficient forward flow pumped, and the patient will collapse and may die if the pump is not restarted immediately.
The prior art and the literature has numerous examples of pumps designed to avoid blood damage, and to avoid thrombus, by the use of sufficiently high flow washing, and the avoidance of excessive shear, which can damage the blood. There is very little prior art dealing with rotary blood pumps adapted to be safe when turned off.
The present invention provides blood pumps that are safe when turned off because they have low regurgitant flow and because the specific pump structure washes out all of the blood contacting surfaces to prevent stagnant regions susceptible to the formation of blood clots.
Left Ventricular Assist devices (LVADs) usually receive their blood inflow from the left ventricular apex, although other connections are sometimes used. The pump provides its outflow to a vascular graft sutured to the aorta. If the pump stops the pressure in the aorta is high enough to cause backflow through the aortic graft and into the left ventricle. This is functionally the same as high regurgitation of the aortic valve. The patient's weakened natural heart will fail rapidly if the backflow is too much. Generally, patients tolerate the reverse flow through the Jarvik 2000, which is less than 1 L/min., and remain conscious and able to function, sometimes for hours. But other clinically applied axial flow LVADs, including the HeartMate II, the MicroMed DeBakey VAD, the Berlin Heart Incor, all have regurgitant flow of over 2 L/min. when turned off. Clinically applied centrifugal pumps including the HeartWare HVAD, the Ventracor, and the Terumo Duraheart also have high regurgitant flow, in excess of 2 L/min. None of these pumps is safe when turned off even briefly in most patients, who quickly go into heart failure and may die.
For all hydrodynamic blood pumps the volume of regurgitant flow is determined by the differential blood pressure across the device and the resistance of the blood flow channels within the pump, attachment cannulae, and grafts. Limitation of regurgitation may be accomplished by the use a flow channel restriction having a relatively small cross sectional area. But for patient safety it is not enough to limit the regurgitant flow to a value below 1 L/min. The pump must also be designed with sufficient washing of all blood contact surfaces under the low flow conditions to prevent thrombus formation.
Centrifugal pumps are especially subject to thrombus formation, because the blood inlet is in the central position, near the axis of rotation of the impeller, and the outlet is tangential located at the largest diameter of the pump chamber in which the impeller rotates. Thus, when stopped, the backflow takes the path of least resistance from the tangential outflow tube to the central inlet opening. Therefore, washout flow of the side of the pump chamber opposite the outlet is relatively poor and stagnant blood is subject to clotting. In centrifugal pump designs using magnetic suspension, hydrodynamic suspension, or partial magnetic with partial hydrodynamic suspension, the pump rotor will crash against the inner walls of the pumping chamber (pump casing) if power is lost, and come to rest in contact with the pump casing. There is so much friction that the rotor cannot be rotated by fluid forces, and the narrow gap regions between the rotor and casing will be especially susceptible to clotting.
This is unfortunate, because centrifugal blood pump designs have some particular advantages compared to axial flow pumps used for VADs. The centrifugal design permits wider gaps between the impeller and casing with lower shear and subsequently lower blood damage. Although red blood cells can withstand high shear for the very brief times occurring in the gaps of axial blood pumps, other types of blood damage, such as fracture of the von Willebrands factor molecule, are thought to be related to shear. Centrifugal blood pumps also have higher efficiency than axials, which is advantageous to increase battery life.
The present invention provides centrifugal blood pumps that are safe to stop, because the rotors are mounted on mechanical bearings and remain free to spin when stopped, and because a unique triple volute or quadruple volute structure is used to provide high washout to all portions of the impeller when the pump is turned off. A screw type inducer or axial flow impeller may be combined with a centrifugal or mixed flow impeller to facilitate back driving of the rotor when power is off. The rotor would then acts like a turbine and may be spun in reverse by the backflow of blood driven by the pressure gradient between the aorta and the pump inlet.
Most centrifugal pumps use a single volute design that results in unbalanced lateral forces on the impeller bearings, due to the non-radial symmetry of the fluid forces. This results from the non-symmetrical geometry of the pump volute. Use of a double volute design, having a splitter blade balances the radial fluid forces and results in more uniform balanced radial forces on the pump bearings. Some centrifugal blood pumps utilize the double volute design to balance the bearing forces. This improves the washout of the impeller when the pump is stopped, but is not optimal. Better washout is obtained by using a triple volute design with two splitter blades, or as many as three or four splitter blades. If too many splitter blades are used, the flow channels become excessively restricted, cause excessive resistance, and may themselves become loci of thrombus. Depending on pump size, triple or quadruple volute designs are optimal.